Otter surveying looks simple until you are standing on a muddy bank at first light, squinting at a dark smear on a boulder and trying to decide whether it is spraint, mink scat or something a fox left behind. That moment of doubt is where most bad records begin. The errors that matter are rarely dramatic; they are small, repeated misjudgements that slowly build a misleading picture of where otters live and how they use a river.

Mistaking another animal's signs for an otter's

This is the most frequent error in the field, and it is entirely understandable. Several mammals leave droppings on stones and logs, and a tired surveyor at the end of a long stretch will see what they hope to see. Slow down at every candidate sign and work through it properly.

  • Otter spraint: usually crumbles easily and is packed with fish bones, scales and occasionally amphibian or bird remains. It is often left on a prominent rock, a tussock, a bridge ledge or a culvert shelf, and the smell is frequently described as sweet, hay-like or faintly of jasmine.
  • Mink scat: often twisted or coiled, frequently on top of a stone, stump or log. It may contain fur, feathers and seeds, and the smell is distinctly unpleasant rather than sweet.
  • Fox or dog: variable in content and often left on paths, trackways and open ground rather than mid-stream boulders.
  • Bird pellets: small, compact, dry and grey, usually found below a perch rather than on a spraint site.

Prints cause similar trouble. Otter prints are broad, several centimetres across, with a wide pad and five toes; webbing is often visible in firm mud. Mink prints are much smaller and neater. And not every cavity in a bank is a holt. Many are rabbit burrows, drainage pipes or simply eroded hollows. Look for supporting evidence — runs, spraint, flattened vegetation — before writing "holt" in your notebook.

Over-reading the evidence

A run of fresh spraints along 200 metres does not mean a run of otters. One animal marking its territory can produce a great deal of sign in a single night, and spraint sites are often used repeatedly by the same individual. Counting signs is not counting animals.

The opposite error is just as common. Otters are secretive, and a clean stretch of river is not proof that otters are absent. Detection is genuinely low, which is why single-visit surveys are weak and repeat visits across seasons are far more reliable. Age your records too: fresh spraint is dark and often glistening, while older material greys, hardens and cracks. Recording month-old sign as current activity distorts the picture, so note the condition alongside the location. If you need a sample, take a small portion and leave the rest in place — it is part of how otters communicate.

Surveying at the wrong time in the wrong conditions

Otters are largely nocturnal and crepuscular across most of the UK, so early morning usually beats mid-afternoon. Sign left overnight is still there, and you are ahead of the walkers, dogs and anglers who may trample it.

Weather matters more than most people expect. Heavy rain washes spraint off rocks within hours. A long dry spell leaves little mud for prints. High water submerges low spraint sites entirely. The best window is often the day or two after a spate, once levels have dropped but before everything has dried out. Note the conditions in your record — a blank result in torrential rain says very little.

Give the job proper time, too. Two kilometres of bank covered in twenty minutes is a walk, not a survey.

Disturbance: the mistake that does real harm

A breeding female with cubs is the most vulnerable animal on the river. Approaching a holt, standing at the entrance, shining a torch inside or sending a dog into a bank cavity can push her out or cause cubs to be moved. Do not enter a holt, and do not place camera traps inside one.

Move quietly along the water's edge, keep groups small, and if an otter appears, stop and let it pass rather than following it. Never pick up a cub found alone; mothers routinely leave cubs while hunting. Otters and their resting places are legally protected across the UK, and disturbing an otter in its holt can be an offence. If any planned work — bridge repairs, bank reinforcement, tree removal — could affect a holt, take advice from the relevant statutory nature conservation body before going near it. For an injured or trapped otter, contact a wildlife rescue or the appropriate authority rather than handling it yourself.

Access, permission and leaving no trace

Find out who owns the bank before you walk it. Angling clubs, farms and estates all have legitimate concerns, and a polite conversation in advance saves a difficult one later. Close gates, keep to the bank, and leave livestock alone.

Resist the urge to pull back vegetation to inspect a suspected lie-up. Trampling creates bare, eroding paths that persist for years, and they lead straight to the very places you are trying to protect. Clean and dry your boots and kit between catchments as a matter of routine, and take every scrap of litter home.

Recording well: the difference between a walk and a survey

A record without context cannot be used. Photograph anything you are unsure of, alongside something for scale — a coin, a ruler, your notebook. Note the grid reference, the time, the substrate, the height of the sign above the water and whether it sat under a bridge or on an open rock. Write down your survey effort as well: the length of bank covered, the time taken and the number of visits. Without that, nobody can interpret your results.

Submit records to your local environmental records centre or county mammal recorder in the format they ask for, and say clearly when you found nothing. Absence data is valuable. If a sign puzzles you, send a photograph to an experienced local recorder before it goes into a database as an otter.

A practical checklist for the next survey

  1. Check the forecast and river levels, and pick your day around them rather than against them.
  2. Confirm access and permission, and carry a map with your grid references marked.
  3. Pack gloves, hand gel, sample bags, a camera with a scale, a notebook and a small torch.
  4. Leave the dog at home, or keep it on a short lead and out of the water.
  5. Walk quietly, scan ahead, and stop at every sign instead of glancing at it.
  6. Record date, time, weather, water level, grid reference, substrate and sign condition.
  7. Photograph anything doubtful, and check it with a local recorder before submitting.
  8. Report injured or dead otters to the relevant authority or your local wildlife trust.

None of this is difficult, and most of it costs nothing but attention. Fewer false records mean better decisions about planning, rivers and protected sites. Less disturbance means otters carrying on with their lives while you do your best to keep up with them.

Photo: MART PRODUCTION / Pexels